The focal length of a mirror does not depend upon the nature of the medium in which it is placed whereas the focal length of a lens depends upon the medium in which it is placed. Thus, there will be no change in the focal length of the concave mirror whereas the focal length of the convex lens will change. If the refractive index of the material of the convex lens is greater than water it will increase, if it is equal to water it will become infinity and if it is less than water it will be negative.
